Those over here who have used the TRS-80 EI with the System 80 make up a 50-40 adapter with a couple of gates. One key to success seems to be putting all the Ram in the System 80 and none in the EI, which cuts down on the traffic between the devices and makes the connection simpler/more reliable.
PMC also made a very rare adapter to achieve the same result. I had one which I gave to Pascal, but the tracks on it were too far corroded for accurate reproduction.
Hope this helps,
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